In heretofore known valves, such as pilot operated, modulating solenoid valves, a phenomenon called cavitation can occur where the valve is utilized to control flow of non-degassed liquids. Cavitation is produced when a liquid has flashed to form local cavities or bubbles which thereafter collapse. Flashing occurs when the pressure of the liquid falls below the vapor pressure, and bubble collapse occurs when the pressure of the liquid returns to a value above its vapor pressure. This process of cavitation causes erosion, noise and impedes liquid flow, as well as producing a drop in efficiency of fluid machinery. It has been found in valve assemblies that the rapid pressure drop across the open valve is at times of such a magnitude that the liquid pressure drops below the vapor pressure and hence results in cavitation. A cavitation condition is of particular concern in pilot operated, modulating valves which operate to temperatures of 1000.degree. F. and pressures of 10,000 psi or greater and have frequent valve open and close cyclic operation. The difficulty of obviating cavitation in pilot operated, modulating valves is that the construction of such valves limits movements to relatively small distances as compared with on-off valves of conventional design. This undesirable cavitation phenomenon is substantially minimized in the valve assembly according to this invention.
It is, therefore, an object of the present invention to provide a valve assembly for the control of liquid flow wherein cavitation is substantially minimized. It is another object of this invention to provide a valve assembly for the control of liquid flow which is less subject to erosion and quieter in operation than heretofore known liquid flow control valves. It is a further object of this invention to provide liquid flow control valve assembly which provides relatively free flow of liquid therethrough.